Presently used RF filters in base stations include mechanically tunable RF filters. These filters are coupled-cavity filters with low-loss, high dynamic range and superior selectivity, but requiring mechanical tuning limits the reconfigurability of the filter in terms of carrier frequency range. What is needed is a filter and signal cancellation system that eliminates the need for mechanical tuning.
Feed-forward configurations of RF discrete-time filters are also available. A known three path RF discrete-time feed-forward filter system is shown in FIG. 1 for improving duplexer isolation. In this figure, h1 and h2 are system blocks containing vector modulators. The symbols labeled τ1 and τ2 are fixed delay lines, which classifies this system as discrete-time.
The known RF discrete-time filters consist of RF paths containing tunable phase shifters and variable attenuators, or vector modulators. The number of RF paths, however, is limited to two or three paths since each additional path requires an additional splitter, combiner, tunable component, gain block and a time delay element. For this reason, only low-order filtering can be performed with known filter systems having two or three paths. What is needed is a two path filter and signal cancellation system that performs high-order filtering, but without needing additional elements such as tunable phase shifters and variable attenuators, or vector modulators for each increase of filter order.
Tunable filters have also been realized in the past with tunable capacitors, such as MEMS, BST and varicap diodes. Heat is dissipated into the tunable components of such known filters, and the Q-factor of tunable capacitors is limited. The limited Q-factor of tunable elements means they cannot be used to filter high-power signals, and large insertion loss results for high-order configurations. What is needed is a filter capable of filtering high-power signals with low insertion loss for high-order configurations.
Known filter banks contain multiple fixed filters and a switch matrix to choose a desired filter configuration. For a large ensemble of filter characteristics, a large number of filters and a large switching matrix is required. The former results in a large space commitment and possesses limited reconfigurability, while the latter results in increasing insertion loss. What is needed is a filter with a small footprint, a continuous range of reconfigurations, and a low insertion loss.
A feed-forward configuration cancellation system with a digital signal processor (DSP) on one path of the filter system has been used to cancel the transmit signal that leaked into the receiver in the transmit passband. The input to the feed-forward system was the digital baseband signal that was also sent to the primary transmitter. The cancellation system is shown in FIG. 2, with this system only the linear portion of the transmitted RF signal can be cancelled. There is a need for a cancellation system that does not only cancel the linear portion of the transmitted RF signal.
Wireless communication systems could include multiband radio architectures, systems capable of spectrum re-farming and software defined radio systems. Common system components in a wireless communication system, such as an IP-based mobile system, include at least one mobile node (or user equipment) and at least one access point AP or a basestation (eNodeB or eNB) on a wireless communication system. The various components on these systems may be called different names depending on the nomenclature used on any particular network configuration or communication system.
For instance, the term “mobile node” includes a mobile communication unit that is called mobile terminal, “smart phones,” or nomadic devices such as laptop PCs with wireless connectivity. A “mobile node” or “user equipment” also encompasses PC's having cabled (e.g., telephone line (“twisted pair”), Ethernet cable, optical cable, and so on) connectivity to the wireless network, as well as wireless connectivity directly to the cellular network, as can be experienced by various makes and models of mobile terminals (“cell phones”) having various features and functionality, such as Internet access, e-mail, messaging services, and the like.
“Mobile nodes” may sometimes be referred to as user equipment, mobile unit, mobile terminal, mobile device, or similar names depending on the nomenclature adopted by particular system providers. A “receiver” and “transmitter” is located at each “access point” (AP), “basestation,” or “user equipment.” As such, terms such as transmitter or receiver in the present invention are not meant to be restrictively defined, but could include components on each mobile communication unit or transmission device located on the network.